Several thousand infants are born in America each year with a unilateral congenital cataract which permits only diffuse light to reach the retina. The cataract can be removed surgically and the affected eye can then be fitted with a contact lens to permit almost normal visual input to the retina. Studies of monocularly deprived animals suggest there is probably a critical period during which this treatment will be maximally effective in restoring vision to what would otherwise be a profoundly amblyopic eye. Our research aims to define that critical period in humans. Many infants are referred to the Hospital for Sick Children annually for treatment of poor vision caused primarily by a unilateral congenital cataract. Surgery is performed at the earliest opportunity but the child's age at referral varies from a few days to over two years. We propose to study the effectiveness of treatment as a function of the timing of the intervention. We propose to study four aspects of the infants' vision: visual acuity, visual fields, optokinetic nystagmus, and the discrimination of form. Studies of monocularly deprived animals suggest early treatment will be necessary for the normal development of the first three but may not be critical for the discrimination of form. We will use optokinetic nystagmus to assess acuity (the finest stripes that elicit OKN) as well as to determine if OKN occurs reliably when the stripes are moving in various directions. We will use the visual preference technique as a second measure of acuity (the finest stripes the infant prefers to grey) and to test the discrimination of form. We will assess visual fields by noting if the infant moves his eyes systematically toward peripheral stimuli. All of these methods have been used previously by at least one of the investigators.